The present invention relates to a constant velocity joint having 8 torque transmitting balls.
Constant velocity joints are classified roughly into the fixed type allowing only angular displacement between two axes and the plunging type allowing angular displacement and axial displacement between two axes. One of the features of the fixed type constant velocity joint, as compared with the plunging type, is that it is capable of taking a high operating angle. For example, the fixed type constant velocity joint used in the drive shaft of an automobile is required to have a maximum operating angle of, e.g., 45xc2x0 or more; however, such high operating angle can be provided only by the fixed type. On the other hand, the fixed type constant velocity joint, as compared with the plunging type, inevitably has its internal construction somewhat complicated.
FIGS. 23A and 23B show a Rzeppa type constant velocity joint typical of the fixed type constant velocity joint. This constant velocity joint comprises an outer joint member 11 having a spherical inner surface 11a axially formed with 6 curved guide grooves 11b, an inner joint member 12 having a spherical outer surface 12a axially formed with 6 curved guide grooves 12b and an inner surface formed with serrations (or splines) 12c for connection to a shaft, 6 torque transmitting balls 13 disposed in ball tracks defined between the guide grooves 11b and 12b of the outer and inner joint members 11 and 12, respectively, and a cage 14 for retaining the torque transmitting balls 13.
The centers A and B of the guide grooves 11b and 12b of the outer and inner joint members 11 and 12, respectively, are offset with respect to the spherical centers of the inner and outer surfaces 11a and 12a, respectively, by an equal distance in opposite directions (the guide groove center A is offset toward the open side of the joint, and the guide groove center B toward the innermost side of the joint). As a result, the ball track defined between the guide groove 11b and the guide groove 12b corresponding thereto is wedge-wise enlarged toward the open side of the joint. The spherical centers of the inner and outer surfaces 11a and 12a of the outer and inner joint members 11 and 12 are located in the joint center plane O including the centers of the torque transmitting balls 13.
When the outer and inner joint members 11 and 12 make an angular displacement of angle xcex8, the torque transmitting balls 13 guided by the cage 14 are maintained in the bisector plane (xcex8/2) bisecting the angle xcex8 irrespective of the value of the operating angle xcex8, and hence uniform velocity is secured.
An object of the present invention is to make this type of constant velocity joint more compact and secure the strength, load capacity and durability which are at least equal to those in a comparative article (such as a 6-ball constant velocity joint as shown in FIG. 23).
To achieve the above object, the invention provides a constant velocity ball joint comprising an outer joint member having a plurality of axially extending curved guide grooves formed in the spherical inner surface thereof, an inner joint member having a plurality of axially extending curved guide grooves formed in the spherical outer surface thereof, a plurality of ball tracks defined between the guide grooves of the outer joint member and the guide grooves of the inner joint member corresponding thereto, said ball tracks being enlarged in one sense of the axial direction, a torque transmitting ball disposed in each of the plurality of ball tracks, a cage having a plurality of pockets for storing the torque transmitting balls, said constant velocity joint being characterized in that the number of said ball tracks and the number of said torque transmitting balls disposed are eight.
The ratio r1 (=PCDBALL/DBALL) of the pitch circle diameter (PCDBALL) of the torque transmitting balls to the diameter (DBALL) of said torque transmitting balls may be within the range 3.3xe2x89xa6r1xe2x89xa65.0. The pitch circle diameter (PCDBALL) of the torque transmitting balls is twice the length of a line segment connecting the centers of the guide grooves of the outer or inner joint member and the centers of the torque transmitting balls (the length of a line segment connecting the centers of the guide grooves of the outer joint member and the centers of the torque transmitting balls and the length of a line segment connecting the centers of the guide grooves of the inner joint member and the centers of the torque transmitting balls are equal), whereby the nature of constant velocity of the joint is secured, said length being hereinafter referred to as (PCR)); thus, PCDBALL=2xc3x97PCR.
The reason for selection of 3.3xe2x89xa6r1xe2x89xa65.0 is that the strength of the outer joint member, the joint load capacity and durability should be made at least as high as in a comparative article (6-ball constant velocity joint). That is, in constant velocity joint, it is very hard to drastically change the diameter (PCDBALL) of said torque transmitting balls in the limited space. Thus, the value of r1 depends mainly on the diameter DBALL of said torque transmitting balls.
If r1 less than 3.3 (mainly when the diameter DBALL is large), the thickness of the other parts (the outer joint member, inner joint member, etc.) would be too small, causing anxiety about the strength. On the contrary, if r1 greater than 5.0 (mainly when the diameter DBALL is small), the load capacity would be too small, causing anxiety about the durability. Also caused is the anxiety that the surface pressure on the surface of contact between the torque transmitting balls and the guide grooves would increase (because the contact oval area decreases with decreasing diameter DBALL), forming a main cause of the chipping of the edges of the guide grooves.
The range 3.3xe2x89xa6r1xe2x89xa65.0 provides greater degrees of strength of the outer joint member, of load capacity and durability of the joint than in the comparative article (6-ball constant velocity joint. This is proved to some extent by tests.
As shown in Table 1 (FIG. 24A) (which shows the estimation of the results of comparative tests), when r1=3.2, sufficient strength for the outer and inner joint members and cage was not obtained, an undesirable result. When r1=3.3, or 3.4, a rather good result was obtained in respect of strength. Particularly, when r1xe2x89xa73.5, sufficient strength for the outer and inner joint members and cage was obtained, a desirable result. In addition, for the range r1 greater than 3.9, though no test has been conducted, it is expected that as good a result as the above will be obtained. If r1 greater than 5.0, however, it is considered that problems will arise in respect of durability and the outer and inner joints, as described above; thus, it is desirable that r1xe2x89xa65.0.
From the above, it is desirable that r1 be in the range 3.3xe2x89xa6r1xe2x89xa65.0, preferably 3.5xe2x89xa6r1xe2x89xa65.0.
Further, in addition to the above arrangement, it is desirable that the ratio r2 (=DOUTER/PCDSERR) of the outer diameter (DOUTER) of the outer joint member to the pitch circle diameter (PCDSERR) of the tooth profile formed in the inner surface of said inner joint member 2 be within the range 2.5xe2x89xa6r2xe2x89xa63.5.
The reason for selection of 2.5xe2x89xa6r2xe2x89xa63.5 is as follows: The pitch circle diameter (PCDSERR) cannot be widely changed because of the relation to the strength of the mating shaft. Therefore, the value of r2 depends of the outer diameter (DOUTER) of the outer joint member. If r2 less than 2.5 (occurring mainly when the outer diameter DOUTER is small), the wall thickness of the each part (outer and inner joint members, etc.,) would be too thin, causing anxiety in respect of strength. On the other hand, if r2 greater than 3.5 (occurring mainly when the outer diameter DOUTER is large), a problem would sometimes arise from a dimensional aspect and the object of making the joint compact could not be attained. The range 2.5xe2x89xa6r2xe2x89xa63.5 provides a greater degree of strength of the outer joint member, of durability of the joint than in the comparative article (6-ball constant velocity joint), and provides contentment in practical use. Particularly, setting 2.5xe2x89xa6r2 less than 3.2 provides the merit of enabling the outer diameter to be reduced as compared with the comparative article (6-ball constant velocity joint of the same nominal size: usually, r2xe2x89xa73.2).
Thus, r2 should be in the range 2.5xe2x89xa6r2xe2x89xa63.5, preferably 2.5xe2x89xa6r2 less than 3.2.
The ball tracks which are enlarged in wedge form in one sense of the axial direction are obtained by offsetting the the centers of the guide grooves of the inner and outer joint members, respectively, with respect to the spherical centers of the outer and inner surfaces thereof axially by an equal distance (F) in opposite directions. It is desirable that the ratio R1 (=F/PCR) of the offset (F) to PCR described above be set within the range 0.069xe2x89xa6R1xe2x89xa60.121.
The reason for selection of 0.069xe2x89xa6R1xe2x89xa60.121 is as follows: When considered with PCR fixed, generally, during application of an operating angle, the greater the offset (F), the lower the track load (which is the load applied to the area of contact between the guide grooves and the torque transmitting balls; therefore, in respect of load, it may be said that larger offset (F) is more advantageous.
If, however, the offset (F) is too large:
(i) torque is reduced in the high operating angle zone, incurring the decrease of allowable load torque;
(ii) in the pockets of the cage, the amount of radial movement of the torque transmitting balls increases, so that to prevent the torque transmitting balls from falling off, it is necessary to increase the wall thickness (radial dimension) of the cage; and
(iii) in the pockets of the cage, the amount of circumferential movement of the torque transmitting balls increases, so that to secure the proper movement of the torque transmitting balls from falling off, it is necessary to increase the circumferential dimension of the cage. Therefore, the posts of the cage become thinner, raising a problem in respect of strength.
On the other hand, if the offset (F) is too small:
(iv) during application of an operating angle, the peak values of the track load (P1) on the load side, and the track load on the non-load side (P2: during 1 revolution, a phase appears in which the non-load side track is loaded) increase, (P1 and P2 indicate peak values at a predetermined phase angle), incurring decreased durability; and
(v) the maximum operating angle decreases.
Thus, too large and too small amounts of offset (F) are both undesirable, and there should be an optimum range in which said problems of (i), (ii), (iii) are balanced with said problems of (iv), (v). However, the optimum range of offset (F) varies with the size of the joint and hence must be determined in relation to the basic size of the joint. This accounts for the use of ratio R1 (=F/PCR). If R1 greater than 0.121, said problems of (i), (ii), (iii) come up and so does said problems of (iv) and (v) if R1 less than 0.069. From the viewpoint of securing the allowable load torque, securing the cage strength, reducing the track load, securing the durability, and securing the maximum operating angle, the optimum range for the offset (F) is 0.069xe2x89xa6R1xe2x89xa60.121. The upper limit (0.121) of the R1 is considerably smaller than the ordinary value of R1 (which is generally 0.14) in the comparative article (6-ball constant velocity joint). It may be said that in respect of the improvement of allowable torque and the cage strength, the present article is given consideration the more for the less R1 as compared with the comparative article. The success of setting the R1 within said range is due to the facts that the present article is provided with 8 torque transmitting balls, which is more advantageous in respect of track load than the comparative article (this is verified by theoretical analysis) and that the temperature rise is relatively low, as compared with the comparative article (this is verified by experiments, see FIGS. 11. and 12). In the comparative article (6-ball constant velocity joint, if R1 is set within said range, the track load would become too high, leading to the decrease of durability.
In addition to the above arrangement, the spherical centers of the outer and inner surfaces of the cage may be offset with respect to the joint center plane including the centers of the torque transmitting balls, axially by the same distance (f) in opposite directions. In this case, it is recommendable that the ratio R2 (=f/PCR) of the offset (f) to PCR be within the range 0 less than R2xe2x89xa60.052.
The reason for selection of 0 less than R2xe2x89xa60.052 is as follows: Generally, the provision of the offset (f) increases the area of the inner surface of the cage, and the resulting decrease of heat generation improves the durability, and allows the increase of the wall thickness of the inlet of the cage incorporating the inner joint member, thus providing the merit of increasing the strength.
However, if the offset (f) is too large,
(i) the amount of circumferential movement of the torque transmitting balls in the pockets of the cage increases, so that in order to secure the proper movement of the torque transmitting balls, the necessity arise of increasing the circumferential dimension of the cage. Therefore, the posts of the cage become thinner, causing a problem in respect of strength; and
(ii) the wall thickness of the portion of the cage opposite to the inlet becomes thinner, causing a problem in respect of strength.
From the above, it is seen that too large offset (t) is not desirable and that there is an optimum range in which the significance of providing offset (f) can be balanced with the problems of (i) and (ii). However, since the optimum range of offset (f) varies with the size of the joint, it should be found in relation to the basic size which indicates the joint size. This accounts for the sue of the ratio R2 (=f/PCR). If R1 greater than 0.052, said problems of (i) and (ii) come up. From the viewpoint of the securing of the cage strength and durability, the optimum range of offset (f) is 0 less than R2xe2x89xa60.052.